Today Chen decided to keep his design simple, and opted for three pieces of stone in contrasting shapes and colors — one for each visible surface of the bookend. For the back two walls of the box, it's best to select stones with a flat side, which you should place completely flush against the aluminum so there's no gap for the concrete to seep into. The bottom surface of the box will be the bottom of the bookend, and doesn't need to be as considered, while the top will have the marble hunk in the foreground emerging from it.